FIG. 4 is a block diagram of an electrical arrangement of a typical and conventional DC regulated power supply 1 used as the AC adapter. In the DC regulated power supply 1 that is a so-called switching power supply, an AC input voltage from a utility commercial power source 2 is first transformed to a DC voltage by a rectifier diode 3 and a smoothing capacitor 4. The AC input voltage from the utility power source 2 becomes the DC voltage of the smoothing capacitor 4. That is, both ends of the smoothing capacitor 4 become a DC power supply. Across the DC power supply (between the both ends of the smoothing capacitor 4) connected is a series circuit of a primary winding 5a of a transformer 5 and a switching element 6, and the switching element 6 is intermittently controlled by a pulse from a control circuit 7. This arrangement induces a voltage in a secondary winding 5b of the transformer 5. The induced voltage is rectified and smoothed by a diode 8 and a smoothing capacitor 9 and then supplied to a load 11 via the power supply lines 10.
Here, for a switching power supply of which a high degree of output voltage accuracy is demanded, the switching power supply (AC adapter) has. an output voltage sensor circuit 12 provided therein, and a voltage at the load end is practically received by the output voltage sensor circuit 12 via output sensing wires 13. Then, a comparator circuit 14 compares a sensing result of the output voltage sensor circuit 12 to a reference voltage from a reference voltage source 15. A comparison result of the comparator circuit 14 is transferred to the control circuit 7 via a photocoupler (not shown), for example, provided for the purpose of electrical isolation. The control circuit 7 changes the pulse duty factor of the switching element 6 depending on output voltage level. With this arrangement, the voltage at the load end is controlled to be a desired voltage.
Further, the output voltage sensor circuit 12 senses an actual voltage at the load end via the output sensing wires 13, not via output end of the AC adapter, thereby realizing a stable output voltage control unaffected by wiring lengths and wire routing of the power supply lines 10, with consideration given to a voltage drop caused by the wiring lengths and wire routing of the power supply lines 10.
However, in the foregoing conventional switching power supply, since the output sensing wires 13 extend closely to the load end, increase in the lengths of the power supply lines 10 results in an unstable control caused by noise and others. Further, in practical usage conditions, a further unstable control might occur since the inductance of cables (output sensing wires 13) changes, for example, for the reason that the output sensing wires 13 are tied together into a ring-like form.
Another conventional switching power supply that can solve such a problem has been proposed in Japanese Laid-Open Patent Application No. 1992/261358 (Tokukaihei 4-261358; published on Sep. 17, 1992). In that conventional switching power supply, a voltage drop occurring in a cable (power supply lines) is corrected, without the aforementioned remote sensing using the output sensing wires 13. Specifically, on the secondary side of the switching power supply, in addition to a load-use direct current transforming section, an auxiliary direct current transforming section is provided. Further, in the conventional switching power supply, the voltage drop occurring in the cable (power supply lines) is corrected by capitalizing on the effects that no load current flows and no voltage drop occurs in the auxiliary direct current transforming section. More specifically, a difference between an output voltage to a load and a DC voltage of the auxiliary direct current transforming section is amplified, and the amplified amount is added to a reference voltage for feedback control, thereby correcting the voltage drop occurring in the cable (power supply lines).
The foregoing conventional switching power supply requires two direct current transforming sections, causing a complicated configuration.